This invention relates to an apparatus for the measurement of light scattered by a particulate within a fluid, and more particularly to a turbidity meter that measures the amount of suspended solids or contaminate in the fluid, particularly for low and very low concentrations of particulate.
Turbidity meters are in wide use today to determine or to indicate the cleanliness of a liquid or a gas by measuring the amount of contaminants therein which are usually in the form of suspended solids. Turbidity meters, such as those to which this invention relates, are not only in common use today for the control of industrial processes, such as determining the cleanliness of gasoline during the refining process, but are also being used for ecological controls, such as measuring the cleanliness of smoke that is allowed to escape from an industrial smoke stack.
As described in U.S. Pat. No. 3,510,666 to Topol, there are basically two types of turbidity meters, one which uses the amount of directly transmitted light and the other which uses the amount of scattered light, either forward or at some selected angle. Utilization of the directly transmitted light involves a light detector which measures the amount of light that is received after transmission through the fluid, and the less transmitted light is received by the detector, the greater is the turbidity. When the turbidity is very low, the light received by the detector is very much the same as the light transmitted, and when the turbidity is very high, the light received by the detector is only a small fraction of the light transmitted. This method has been found unsuitable for the measurement of low particulate concentration because the transmitted and received light are almost equal and therefore the sensitivity is very low. The same is true for high concentrations where hardly any light is received.
The scattered light method has a light sensitive detector positioned along any axis on which scattering is to be measured and which is shielded from the direct light transmitted along a transmission axis in case the forward scattered light is to be measured. The light measured is actually the light that is scattered by the particulate, and as the concentration of the particulate increases, the amount of scattered light likewise increases until the concentration becomes so high that intervening particles make the measure difficult because there would be secondary scattering. This method is preferred for low particulate concentration.
One of the difficulties with scattering type turbidity meters of the prior art, particularly if the particulate concentration is low, has been their sensitivity to light signals other than those due to scattering, such as internal reflections of the illuminating beam of light and ambient light which could reach the detector. When the particulate concentration is very low, say from 0.1 to 100 Formazin units, the error signals due to internal reflection and to ambient lighting have been found to be approximately of the same order of magnitude as the scattered light, which therefore causes substantial errors.